Add template/layout/view variable capturing to integrationtestcase

Abuse the event hooks to capture interesting data about the request and
store it as testcase properties so assert's can be built later on.
Add asserts for template/layout.
Add assertions for flash/session/cookies.

Add additional assertions and accessors for view variables.
Fix doc block mistakes.
Remove assertFlash()

A few people have voiced concerns that it will feel unreliable as the
rendered view will not have flash messages to assert against. I would
rather we have fewer more stable asserts and avoid confusing/problematic
methods entirely.
Merge pull request #4564 from cakephp/integration-test-case-2

Integration test case part 2